Pulse frequency modulation (PFM) method is known, in addition to pulse width modulation (PWM) method, as switching power supplies. A switching power supply of the PWM method adjusts the output voltage by controlling the ON/OFF time ratio (duty) of a switching element after obtaining comparator output by changing the threshold level of the comparator in accordance with an output voltage using a chopping wave signal synchronized with a clock. The switching power supply of the PWM method has widely been used, but is basically based on feedback control, which always poses a problem of stability (oscillation).
A switching power supply of the PFM method, by contrast, includes a choke coil that stores/releases current energy in conjunction with an ON/OFF operation of a switching element and a capacitor that forms an output voltage by charging and adjusts the output voltage by exercising the ON/OFF control of the switching element directly based on comparator output that compares the output voltage with a predetermined voltage and thus has advantageously no stability problem described above posed for the PWM method.
However, in a switching power supply of the PFM method, the switching element is in principle 100% duty that continues the ON operation in a period in which the output voltage is below the predetermined voltage and conversely 0% duty that continues the OFF operation in a period in which the output voltage is above the predetermined voltage. The former case occurs when the power supply is started up or a light load or no load is switched to a heavy load. The latter case occurs when a heavy load is switched to a light load or no load. That is, the switching power supply of the PFM method has a weakness that the switching frequency changes significantly when operating conditions change such as when the load abruptly changes.
Thus, there has been a problem that higher harmonics may not be efficiently be removed because, as described above, the fundamental frequency of the switching frequency changes significantly depending on operating conditions even if measures are taken such as exercising duty control in such a way that the switching frequency falls within a predetermined frequency range in which a frequency of generated higher harmonics thereof does not act as a disturbing wave to prevent disturbances by higher harmonics generated by the ON/OFF operation of a switching element or providing a harmonic suppressor filter.